1. Field of the Invention
The present invention relates to a process for preparing iodoalkynylcarbamates having a low tendency of discoloring or yellowing when exposed to light. More specifically, the invention relates to a process for preparing iodoalkynylcarbamates without employing trialkylamine catalysts.
2. Related Art
Latex paint, in particular white or light colored paint for outdoor application, is subject to discoloration by fungal (mold and mildew) and algae growth. A wide variety of preservatives have been developed to prevent such discoloration. One well-known group of preservatives is derived from iodopropargyl-containing chemical compounds.
Iodopropargyl-containing chemical compounds are known based upon various chemical function groups, such as ethers, esters, etheresters, amides, alcohols, pyrazoles, triazoles, pyridines, aminoacidesters, benzoxazoles, and carbamates. Carbamates have shown good market potential and are commonly used as film fungicides, mold and mildew prohibitors, slime preventing additives, algacides, wood preservatives, cosmetic additives, and disinfectors. Aryl derivatives of carbamates seem to be more suitable for agricultural use than alkyl derivatives. Alkyl derivatives have the general formula: EQU IC.tbd.C--CH.sub.2 --O--CONHR
where R is H or a linear or branched alkyl group having 1 to 20 carbon atoms. The two hydrogen atoms at the propargyl group can be substituted by an alkyl group having 1 to 6 carbon atoms.
Alkyl and aryl carbamates are commonly prepared by reacting alcohol with isocyanate to form the carbamate, as described for preparing urethanes in Organic Synthetic Chemistry, Vol. 19, No. 11, pp. 775-789 (1961).
Propargylcarbamates are traditionally synthesized according to the following equation: EQU HC.tbd.C--CH.sub.2 OH+RNCO.fwdarw.HC.tbd.C--CH.sub.2 OCONHR
where R is an unsubstituted or substituted, saturated or unsaturated, alkyl, cycloalkyl, branched or unbranched carbon chain containing 1 to 20 carbon atoms.
It is common practice to accelerate such reactions with a suitable catalyst or combination of catalysts. Isocyanates are very reactive and react with almost any compound having an active hydrogen atom. They will react with an alcohol to form urethanes and with amines to form substituted ureas. Isocyanates will also react with water to form carbamic acid and decompose readily to carbon dioxide and the amine. In a secondary reaction, the amine will form disubstituted urea. Furthermore, a variety of crosslinking reactions take place depending on reaction conditions and the use of different kinds of catalysts. The structure of the alcohol and the isocyanate also plays an important role in directing the reaction which includes, for example, urethane-isocyanate, urea-isocyanate, and trimerization reactions. Even polymerization is known. These phenomenons take place over time and deactivate to a certain degree the reactivity of isocyanates.
According to prior art methods of forming carbamates, where an extended period of time is expected to pass between the synthesis of isocyanate and the use of the isocyanate to form carbamate, it is necessary to carefully select appropriate catalysts and catalyst combinations to drive the reaction forming the carbamate. These catalysts are used to start the reaction, decrease reaction time, and to drive the reaction in the desired direction. They are also used to overcome deactivations and competition reactions.
During the past 30 years, this type of reaction and the efficiency of tertiary amines as catalysts for the reaction, including variations according to basicity and steric hindrance of tertiary amines, have been studied. For example, Polymer Technologies Inc., a subsidiary of the University of Detroit, has conducted such a study, as well as others.
The catalytic activity of tertiary amines generally increases as the basicity of the amine increases and the steric shielding of the nitrogen in the amine decreases. Low chain alkylamines or alkylarylamines, such as triethylamine, dimethylcyclohexylamine, or dimethylbenzylamine, are usually preferred over triethylenediamine, which has extreme shielding. For example, use of low chain ethylene-containing amines as catalysts promotes urethane and urea reactions equally. In contrast, catalysts with propylene-containing groups promote the urethane reaction over the urea reaction. Catalysts with synergistic properties are derived from the group of transition metal organic compounds as well as from tin and antimony. Examples of such catalysts are dibutyltindilaurate, stannous octoate, tetrabutyltin, dibutyltinchloride, dibutyltindioleate, and equivalent lead compounds.
Since urethanes of unknown alcohols form specific physicochemical parameters, this reaction is also used to identify alcohols, preferably using phenylisocyanate to form carbamates of specific melting points, as described in "Organikum," a basic organic chemistry course, VEB Deutscher Verlag der Wissenschaften, Berlin 1967.
Reaction of halogenated derivatives of propargyl alcohol with isocyanates is described in Japanese Patent No. 3903 to Meiji Con. Co. Ltd. and U.S. Pat. No. 3,923,870 to Singer. The specific melting points of carbamates provide a convenient method for identifying individually prepared carbamates. Examples of melting points specific for individual carbamates are listed in Table I, below
TABLE I ______________________________________ CARBAMATE MELTING POINT ______________________________________ Methylphenylurethane 47.degree. C. Ethylphenylurethane 52.degree. C. Isopropylphenylurethane 88.degree. C. Propylphenylurethane 57.degree. C. Alkylphenylurethane 70.degree. C. n-Butylphenylurethane 61.degree. C. n-Hexylphenylurethane 42.degree. C. Cyclohexylphenylurethane 82.degree. C. Propargyl-2,4-dichlorophenylurethane 73.degree. C. Propargyl-2,5-dichlorophenylurethane 75.degree. C. Propargyl-2-chloro-6-methylphenylurethane 102.degree. C. Propargyl-2-chloro-4-methylphenylurethane 60.degree. C. Propargylpentylnitrilurethane 55.degree. C. ______________________________________
In the event that the urethane is a liquid at ambient temperature, it is more appropriate to characterize the urethane by its specific properties, such as its refractive index, as listed below in Table II.
TABLE II ______________________________________ REFRACTIVE CARBAMATE INDEX ______________________________________ Propargyl-2,2-dimethylpropylurethane n.sup.20.sub.D = 1,4561 Propargyl-n-butylurethane n.sup.20.sub.D = 1,4560 Propargyl-i-butylurethane n.sup.20.sub.D = 1,4576 Propargyl-1-ethylpropylurethane n.sup.20.sub.D = 1,4572 Propargylchlorohexylurethane n.sup.20.sub.D = 1,4859 Propargyltrifluoromethylcyclohexylurethane n.sup.20.sub.D = 1,4450 ______________________________________
Iodination of the final urethanes can be accomplished either by (1) iodinating the available commercial propargyl alcohol according to standard methods, such as those described in Journal of the American Chemical Society, 102:4193-4198 (1980) and U.S. Pat. No. 3,923,870 to Singer, or (2) first preparing the corresponding propargylurethanes and then iodinating according to various methods, such as those described in EP 14032, DE 3921035, EP 539092, EP 513541, and U.S. Pat. No. 4,841,088 to Kusaba et al.
The resulting iodopropargyl derivatives have specific melting points which provide a convenient method for distinguishing between the derivatives. The derivatives listed below in Table III are all solids.
TABLE III ______________________________________ MELTING IODOPROPARGYLURETHANE DERIVATIVE POINT ______________________________________ Idopropropargyl-m-chlorophenylurethane 75.degree. C. Iodopropargylphenylurethane 145.degree. C. Iodopropargyl-3-nitrophenylurethane 151.degree. C. Iodopropargyl-4-nitrophenylurethane 170.degree. C. Iodopropargyl-3-methoxyphenylurethane 108.degree. C. Iodopropargylmethylurethane 56.degree. C. Iodopropargylbutylurethane 67.degree. C. Iodopropargyl-t-butylurethane 84.degree. C. Iodopropargylcyclohexylurethane 120.degree. C. Iodopropargyldodecylurethane 56.degree. C. ______________________________________
All iodopropargyl urethanes have fungicidal properties. However, halo-organic derivatives are subject to photo-oxidation when exposed to light. Photo-oxidation is a natural result of entropy which promotes the reassimilation of carbon back into the carbon cycle. In particular, halo-substituted organic compounds are likely to decompose when exposed to sunlight. Organic halogenic compounds, particularly chlorine, bromine, and iodine organics, form fragments of free radicals following the absorption of ultraviolet (UV) light.
Iodoorganic compounds, depending on the intensity, wavelength, and exposure time of UV light, form elemental iodine as well as other free radical fragments following UV absorption. Elemental iodine, like bromine, is yellow to brown in color. Thus, UV light exposure causes discoloration and yellowing in iodoorganic compounds as they undergo photo-oxidation and form elemental iodine and other free radical fragments. This phenomenon is exhibited by iodopropargylbutylurethane, also known as 3-iodo-2-propynylbutyl carbamate, abbreviated "IPBC" a common preservative used in paint compositions.
There are several prior art methods for preventing discoloration caused by formation of elemental iodine in response to absorption of UV light, with varying levels of success. Various research groups have discovered that the higher the iodine ratio in an iodine organic compound, the more likely the tendency to form radicals, and eventually elemental iodine, under UV light radiation. This is expected for di- and particularly tri-iodoalkyl alcohol as well as triiodoalkyl derivatives. Of all the tested materials, triiodoalkylalcohol breaks down the easiest through photoirradiation.
One known approach to avoiding discoloration and yellowing is the addition of up to 20% by weight of an organic epoxide stabilizer which apparently functions as a hydrogen iodide acceptor. These epoxy-based acid scavengers include epoxides of vegetable oils and fats, aliphatic resins, cycloaliphatic resins, and aryl resins. Also, epoxy derivatives, such as propylene oxide, styrene oxide, butylene oxide, and epichlorhydrin can be used. Various examples are described in U.S. Pat. No. 4,297,258 to Long, Jr. This method has only been mildly successful in preventing discoloration and yellowing.
A second known approach for preventing discoloration employs epoxides as color stabilizers for iodoalkynyl carbamate fungicides in paint compositions and coatings. Such coatings are prepared incorporating epoxides, such as 25% epichlorohydrin based on the amount of carbamate. As stated in U.S. Pat. No. 4,276,211 to Singer et al., this method of preventing discoloration has a color rating of 2 on a scale of 1 to 5 (1=white, 5=dark yellow), as opposed to a rate of 5 without epoxide.
A third method for preventing photochemical breakdown and resulting discoloration employs UV-stabilizers of the chemical nature of triazoles or hindered amines. Examples of both classes are benzotriazole and bis-(1,2,2,6,6-pentamethyl-4-piperdinyl)(3,5-butylpropanedioate) (see Peter D. Gabriel and Robert M. Iannucci, "Protection of Mildewcides and Fungicides From Ultraviolet Light Induced Photo-Oxidation," Journal of Coatings Technology, Vol. 56, No. 712, pp. 33-38 (May 1984)). According to thin layer chromatography and bioassay analysis, photochemical breakdown is prevented by using 0.5% to 2% stabilizing agent.
The above-mentioned prior art methods require one or more catalysts to form carbamate and require additional treatment, for example use of additives, to prevent discoloration and yellowing of materials incorporating the carbamate due to photo-oxidation. It is therefore desirable to provide an economical, effective method for preparing carbamates that eliminates or reduces the need for a catalyst and prevents photochemical breakdown and resulting discoloration without requiring additional treatment.